Trying to get early (~2021 March) blocks either with public RPC, QuickNode RPC or Alchemy RPC results in almost 1/3 blocks being missing (not skipped!). Why is that so? Can those blocks be retrieved from somewhere else? And does this happen in more recent slots as well?
It's up to the RPC node operator to decide how much historical ledger data they will serve. I don't think many serve very old data. It's a large amount of data to store and the demand for this data is low, so RPC node resources are typically dedicated to more recent data.
The full history of the block chain is stored in at least one place: a Google bigtables instance that the Solana Foundation manages. It is not available to the public, you have to get special permission to access it from the Foundation.
At least one RPC node operator has made a duplicate of this full history, GenesysGo. I have tried some sample queries and it appears to me that the full block history is available from their RPC node.
Also, the block you specifically mentioned, 80006252, was skipped and so is not available from any source.